Ocean biodiversity and the future of seafood: Take 2

In November 2006, fourteen marine scientists and resource economists (including yours truly) led by Boris Worm of Dalhousie University published a paper in Science documenting the functional importance of marine biological diversity and linking it in particular to sustainability of fisheries. The paper generated global press coverage (the breadth of which is illustrated by its appearance on both Fox News and Al Jazeera), primarily because of its suggestion that, if current trends continue, most of the world’s fisheries could be headed for collapse by mid-century.The paper also, predictably, generated a vehement backlash from some quarters. Alas, many of them appear to have paid more attention to the press release, which headlined with projected seafood collapse, than the paper itself, which focused on links between biodiversity and ecosystem services such as seafood production.

Today’s issue of Science publishes three of the critiques of the original Worm et al. paper, and allowed us to respond (you can read the whole shebang here). Separately, in the online “Technical comments” section of Science, we respond to three additional criticisms involving nuances of the statistical analysis.

Steven Murawski et al., of the National Marine Fisheries Service, object that fishery catch data, such as those used in our original paper, do not accurately represent the biomass of a fish “stock” (i.e., population), particularly if the population is being closely managed, for example in a rebuilding plan in which fishing is restricted. This is clearly true under certain circumstances – biomass and catch may be poorly correlated for any individual stock, as exemplified by the single stock (Georges Bank haddock) described by Murawski et al. However, our paper did not purport to characterize biomass of individual stocks. Rather we discussed a global trend, assuming that such individual variation will be overwhelmed by the trend over large numbers of stocks.

And that appears in fact to be true: when we use only those fish stocks for which good data are available on both biomass and catch, the trends look pretty similar, although the biomass trend is noiser because of the sparser data (see figure panel B: black and red symbols are data based on catch and biomass, respectively). Indeed, even for the Georges Bank haddock stock that Murawski et al. discuss, catch and stock biomass were closely correlated until the recent large-scale closure of the fishery (see figure, panel A: white and red symbols are data based on catch and biomass, respectively). Bottom line? For this purpose, fishery catch data do not seem to invalidate our original conclusions. Indeed one might argue that this example illustrates the pitfalls of extrapolating conclusions from a single fish stock considered in isolation.

Then there is the critique by Ray Hilborn (of “Faith-based fisheries” fame), who comes out with both barrels characteristically blazing. Hilborn also takes us to task for, among other things, using catch data to discuss prospects of fishery depletion. This is a bit ironic, given that he and co-authors used catch data in a similar fashion in their own review in 2003 and reached the quite similar conclusion that “we anticipate further declines in abundance, further loss of jobs and fishing communities, and potential structural change to marine ecosystems.” It is worth reiterating here that our original paper was not intended to explain the causes of particular fisheries declines, globally or locally, but to test the potential role of marine biodiversity in stabilizing fisheries catches and other goods and services provided by marine ecosystems. There has been little dispute about that connection.

In a nutshell (or dare I say, in a clamshell), the criticisms raised do not invalidate the main conclusions of the original analysis by Worm et al. Fish stocks have declined worldwide over the last few decades, as widely recognized and documented not only in our paper but by the comprehensive assessment of the United Nations Food and Agricultural Organization. What is new is our documentation that biodiversity is critical to maintaining the normal functioning of marine ecosystems, and the goods and services that they provide to human society, including the productivity and resilience of global fishery catches.

Despite some initial acrimony about details, it’s clear that most fisheries biologists well recognize that diverse, structurally complex ocean ecosystems are important to sustainably productive fish populations fish. This recognition is evidenced by mandates to protect essential fish habitat and to implement ecosystem-based management in the recently reauthorized Magnuson-Stevens Fishery Conservation and Management Act. Thus, the question is no longer whether biodiversity and fisheries are declining but how to restore them. So what to do? Our take:

“We are not advocating the establishment of MPAs as the sole policy solution to resolving the problems of fisheries management. Rather, we suggest that sustainable fisheries practices, pollution control, maintenance of essential habitats, and creation of marine reserves all need to be part of a broader strategy to manage ocean ecosystems for their biodiversity and associated services.”

Food for Thought

“Political activity of any kind is doomed to the superficiality and temporariness of politics, able only to produce generalizations that will hold conflicting interests uneasily together for a time. Bu the life that attaches itself to the earth, to fulfill itself in the earth's meanings and demands, though it will certainly affect politics, will affect the earth and the earth's life even more.” Wendell Berry